Power steering device and power steering device control unit

ABSTRACT

Disclosed is a power steering device having at least two redundant steering torque sensor units TS 1  and TS 2 , at least two redundant steering angel sensor units AS 1  and AS 2 , at least two redundant motor position sensor units  61  and  62  and a control unit configured to: in a normal state, perform steering assist control based on a steering torque detection signal of one of the steering torque sensor units TS 1  and TS 2 , a steering angle detection signal of one of the steering angle sensor units AS 1  and AS 2  and a motor position detection signal of one of the motor position sensor units  61  and  62 ; perform redundant monitoring on the steering torque sensor units TS 1  and TS 2 , the steering angel sensor units AS 1  and AS 2  and the motor position sensor units  61  and  62 ; and, upon detection of an abnormality in any sensor output signal by the redundant monitoring, switch the abnormal signal to an alternative signal. It is possible by this control process to preserve the steering assist function even in the occurrence of the abnormality in the sensor output signal.

FIELD OF THE INVENTION

The present invention relates to a power steering device and a powersteering device control unit.

BACKGROUND ART

With the recent widespread use of EPS devices, further improvement ofproduct appeal is desired. It is particularly demanded to, in the eventof a failure in an EPS device, preserve the steering assist function ofthe EPS device even for a limited time.

Patent Document 1 discloses a technique to preserve the steering assistfunction of the EPS device by, upon detection of an abnormality in atorque sensor, generating an alternative signal based on signals fromupstream and downstream sides of a torsion bar, switching an outputsignal of the torque sensor to the alternative signal and continuingsteering assist control operation according to the alternative signal.

PRIOR ART DOCUMENTS Patent Document

Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-206070

SUMMARY OF THE INVENTION

As mentioned above, it has been a problem for the power steering deviceand its control unit to preserve the steering assist function in theevent of an abnormality in the sensor output signal.

The present invention has been made in view of the above problem.

In accordance with one aspect of the present invention, there isprovided a power steering device having at least two redundant steeringtorque sensor units, at least two redundant steering angel sensor units,at least two redundant motor position sensor units and a control unitconfigured to: in a normal state, perform steering assist control basedon a steering torque detection signal of one of the steering torquesensor units, a steering angle detection signal of one of the steeringangle sensor units and a motor position detection signal of one of themotor position sensor units; perform redundant monitoring between onsteering torque sensor units, the steering angel sensor units and themotor position sensor units; and, upon detection of an abnormality inany sensor output signal by the redundant monitoring, switches theabnormal signal to an alternative signal.

It is therefore possible for the power steering device and powersteering device control unit according to the present invention topreserve the steering assist function even in the event of theabnormality in the sensor output signal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a power steering device according to afirst embodiment of the present invention.

FIG. 2 is a block diagram of an electric system of the power steeringdevice according to the first embodiment of the present invention.

FIG. 3 is a schematic diagram showing inputs and outputs of a steeringtorque sensor and a steering angle sensor of the power steering deviceaccording to the first embodiment of the present invention.

FIG. 4 is a flowchart of an abnormal signal switching process of thepower steering device according to the first embodiment of the presentinvention.

FIG. 5 is a control block diagram for the abnormal signal switchingprocess of the power steering device according to the first embodimentof the present invention.

FIG. 6 is a schematic view showing an example of calculation of anoperational steering torque signal (as an alternative signal) in thepower steering device according to the first embodiment of the presentinvention.

FIG. 7 is a flowchart of an abnormal signal switching process of a powersteering device according to a second embodiment of the presentinvention.

FIG. 8 is a control block diagram for the abnormal signal switchingprocess of the power steering device according to the second embodimentof the present invention.

FIG. 9 is a schematic view showing an example of calculation of anoperational steering angle signal (as an alternative signal) in thepower steering device according to the second embodiment of the presentinvention.

FIG. 10 is a flowchart of an abnormal signal switching process of apower steering device according to a third embodiment of the presentinvention.

FIG. 11 is a control block diagram for the abnormal signal switchingprocess of the power steering device according to the third embodimentof the present invention.

FIG. 12 is a schematic view showing an example of calculation of anoperational motor position signal (as an alternative signal) in thepower steering device according to the third embodiment of the presentinvention.

FIG. 13 is a flowchart of an abnormal signal switching process of apower steering device according to a fourth embodiment of the presentinvention.

FIG. 14 is a flowchart of an abnormal signal switching process of apower steering device according to a fifth embodiment of the presentinvention.

FIG. 15 is a time chart of an average signal generated in the powersteering device according to the fifth embodiment of the presentinvention.

FIG. 16 is a flowchart of an abnormal signal switching process of apower steering device according to a sixth embodiment of the presentinvention.

FIG. 17 is a control block diagram for the abnormal signal switchingprocess of the power steering device according to the sixth or seventhembodiment of the present invention.

FIG. 18 is a graph showing one example of steering assist limitprocessing in the power steering device according to the sixthembodiment of the present invention.

FIG. 19 is a graph showing another example of steering assist limitprocessing in the power steering device according to the sixthembodiment of the present invention.

FIG. 20 is a flowchart of an abnormal signal switching process of thepower steering device according to the seventh embodiment of the presentinvention.

FIG. 21 is a flowchart of an abnormal signal switching process of apower steering device according to an eighth embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, power steering devices and their control units according tofirst to eighth embodiments of the present invention will be describedin detail below with reference to FIGS. 1 to 21.

First Embodiment

FIG. 1 is a schematic view of the power steering device according to thefirst embodiment of the present invention.

As shown in FIG. 1, the power steering device has a basic steeringsystem including a steering wheel (not shown), a steering shaft 1, apinion shaft 2 and a rack shaft 3 so as to, when the steering wheel isturned by a vehicle driver, transmit a steering torque of the steeringwheel to the pinion shaft 2 via the steering shaft 1, convert therotation of the pinion shaft 2 to linear movement of the rack shaft 3and thereby steer left and right steerable vehicle wheels (not shown)connected to both ends of the rack shaft 3. Namely, the rack shaft 3 hasrack teeth engaged with the pinion shaft 2 such that the engagement ofthe rack teeth and the pinion shaft constitutes a conversion mechanismto convert the rotation of the steering shaft 1 to steering operation.

A steering torque sensor TS (such as resolver) is disposed on a housingof the pinion shaft 2 to detect a steering angle of the steering wheel.A motor position sensor 6 (such as resolver or IC) is disposed to detecta rotor rotational position of an electric motor M. Based on outputsignals of the steering torque sensor TS and the motor position sensor 6and vehicle speed information, the electric motor M is driven andcontrolled by a motor control circuit module (not specifically shown) ofthe control unit (hereinafter also referred to as “ECU”) so as to applya steering assist torque from the electric motor M to the rack shaft 3via a reduction gear 5.

As the reduction gear 5 is disposed on an output shaft of the electricmotor M, the torque of the electric motor M is reduced by the reductiongear 5 and converted to the linear movement of the rack shaft 3.

Herein, the steering shaft 1 has two axially separable shaft members: aninput shaft member on the steering wheel side and an output shaft on therack shaft 3 side. The input and output shaft members are coaxiallycoupled together via a torsion bar (not shown) and are rotatablerelative to each other by torsion of the torsion bar. In the firstembodiment, the steering torque sensor TS has each sensor unitconstituted by first and second angle sensor elements to detect arotational angle of the input shaft member and a rotational angle of theoutput shaft member, respectively, calculate the amount of torsion ofthe torsion bar based on output signals of the first and second anglesensor elements and determine the steering torque according to thecalculated torsion amount.

A steering angle sensor AS (such as MR element or IC element) is alsodisposed on the torsion bar.

FIG. 2 is a block diagram of an electric system of the power steeringdevice. FIG. 3 is a schematic diagram showing inputs and outputs of thesteering torque sensor TS, the steering angle sensor AS and the motorposition sensor 6.

As shown in FIGS. 2 and 3, two main and sub steering torque sensor unitsTS1 and TS2 each having the above-mentioned first and second anglesensor elements, two main and sub steering angle sensor units AS1 andAS2 and two main and sub motor position sensor units 61 and 62 arearranged to detect the steering torque, the steering angle and the motorposition. Steering torque detection signals Tt(Main) and Tt(Sub),steering angle detection signals θs(Main) and θs(Sub) and motor positiondetection signals θm(Main) and θm(Sub) are outputted from the steeringtorque sensor units TS1 and TS2, the steering angle sensor units AS1 andAS2 and the motor position sensor units 61 and 62 to torque signalreceiver 21 b and 21 d, steering angle signal receivers 21 a and 21 cand motor position signal receivers (not specifically shown) of the ECU4, respectively.

A power supply circuit 7 is provided as a power source to supply powerto the sensors, MPU 9, ICs and the like. A CAN communication circuit 8is provided to perform data/information communication with the vehicle.The MPU 9 is configured to perform various processing for EPS assistcontrol, motor current control, functional element abnormalitydetection, safe mode transition etc. A fail-safe circuit 13 is providedto, when the MPU 9 detects any abnormality and decides on the necessityfor system shutdown, perform a motor current source shutdown functionaccording to a command from the MPU 9.

A drive circuit 10 is provided to drive an inverter circuit 12 accordingto a command from the MPU 9. The inverter circuit 12 is equipped with adriver element and driven according to a command from the drive circuit10. By the current flow from the inverter circuit 12, the electric motorM is driven to output the motor torque for steering assist. A currentsensor 11 a is provided as a current detection element to detect thecurrent at the downstream side of the inverter circuit 12. A currentmonitor circuit 11 b is arranged to monitor whether the motor commandcurrent is supplied as targeted to ensure the torque of the electricmotor M required for steering assist control.

For control of the motor, main and sub current detection circuits 14 aand 14 b are provided to detect the current after high-responsefiltering. For monitoring of overcurrent in the inverter circuit 12,main and sub current detection circuits 15 a and 15 b are provided todetect the average current after low-response filtering.

Next, an abnormal signal switching process executed by the ECU 4according to the first embodiment will be explained below with referenceto FIGS. 4 to 6. FIG. 4 is a flowchart of the abnormal signal switchingprocess. FIG. 5 is a control block diagram of the control unit. FIG. 6is a schematic view showing an example of calculation of an operationalsteering torque signal.

At step S1 a, the steering torque detection signals Tt(Main) and Tt(Sub)are retrieved from the main and sub steering torque sensor units TS1 andTS2, respectively. At step S2 a, the steering angle detection signalsθs(Main) and θs(Sub) are retrieved from the main and sub steering anglesensor units AS1 and AS2, respectively. At step S3 a, the motor positiondetection signals θm(Main) and θm(Sub) are retrieved from the main andsub motor position sensor units 61 and 62, respectively.

At step S4 a, a calculated steering torque signal (alternative signal)Tts(Main) is obtained by a first steering torque calculation circuit 16a of an alternative signal calculation section 20 based on the steeringangle detection signal θs(Main), the motor position detection signalθm(Main), the torsional rigidity Ktb of the torsion bar and thereduction ratio Ng between the pinion shaft 2 and the motor shaft.

A method of calculation of the calculated steering torque signal(alternative signal) Tts(Main) will be explained below with reference toFIG. 6. In the first embodiment, the calculated steering torque signal(alternative signal) Tts(Main) is obtained by multiplying the relativeangle between upstream and downstream sides of the torsion bar by thetorsional rigidity Ktb of the torsion bar. The steering angle detectionsignal θs(Main) is used as the angle of the upstream side of the torsionbar. The angle of the downstream side of the torsion bar (i.e. theangular position of the pinion shaft 2) is determined by multiplying themotor position detection signal θm(Main) by the reduction ratio Ngbetween the pinion shaft 2 and the motor shaft. Namely, the calculatedsteering torque signal Tts is given by the following equation (1).

Tts=Ktb×(θs−θp)  (1)

Next, a calculated steering torque signal (alternative signal) Tts(Sub)is obtained by a second steering torque calculation circuit 16 b of thealternative signal calculation section 20 based on the steering angledetection signal θs(Sub), the motor position detection signal θm(Sub),the torsional rigidity Ktb of the torsion bar and the reduction ratio Ngbetween the pinion shaft 2 and the motor shaft.

A method of calculation of the calculated steering torque signal(alternative signal) Tts(Sub) is similar to that of the calculatedsteering torque signal (alternative signal) Tts(Main).

The calculated steering torque signal Tts(Main) is determined as thealternative signal by an alternative signal determination circuit 18 inthe first embodiment.

At step S5 a, it is judged by an abnormality detection section 17 awhether the difference |Tt(Main)−Tt(Sub)| between the two steeringtorque detection signals is smaller than an abnormality detectionthreshold value. When the difference |Tt(Main)−Tt(Sub)| between the twosteering torque detection signals is judged as being smaller than theabnormality detection threshold value, the process goes to step S6. Atstep S6, the abnormality counter is cleared by the abnormality detectionsection. At step S7 a, the steering torque detection signal Tt(Main) isset as the signal for steering assist control by a switching judgmentsection 19. After that, the process ends the current control cycle.

When the difference |Tt(Main)−Tt(Sub)| between the two steering torquedetection signals is judged at step S5 a as being greater than or equalto the abnormality detection threshold value, the process goes to stepS8. At step S8, it is judged by the switching judgment section 19whether the abnormality counter is greater than or equal to apredetermined value (abnormality determination time period). When theabnormality counter is judged as being greater than or equal to thepredetermined value, the process goes to step S9. At step S9, thealternative signal is selected and set as the signal for steering assistcontrol by the switching judgment section 19. At step S10, a warninglamp is turned on so as to give a warning to the vehicle driver.

When the abnormality counter is judged at step S8 as being smaller thanthe predetermined value, the process goes to step S11. At step S11, theabnormality counter is incremented by the abnormality detection section.Herein, the abnormality counter is incremented every control cycle whenthe condition in which the difference |Tt(Main)−Tt(Sub)| between the twosteering torque detection signals is greater than or equal to theabnormality detection threshold value continues for the abnormalitydetermination time period.

As explained above, the power steering device of the first embodiment isadapted to obtain the calculated steering torque signal (alternativesignal) Tts(Main), Tts(Sub) using the steering angle detection signalθs(Main), θs(Sub) and the motor position detection signal θm(Main),θm(Sub) and, in the event of an abnormality in at least one of thesteering torque detection signals Tt(Main) and Tt(Sub), utilize thecalculated steering torque signal for steering assist control in placeof such an abnormal detection signal. Accordingly, it is possible tocontinue steering assist control operation based on the calculatedsteering torque signal (alternative signal) Tts(Main), Tts(Sub) andthereby possible to reduce the steering load of the vehicle driver. Itis also possible to decrease fluctuations in steering assist forcecaused due to the low accuracy of the alternative signal.

Furthermore, both of the steering torque detection signals Tt(Main) andTt(Sub) are outputted as torque signals of the steering shaft. It isthus possible to easily compare these signals and enable early andaccurate detection of the abnormality.

Second Embodiment

Although the first embodiment is characterized by the generation of thealternative signal upon detection of the abnormality in the steeringtorque detection signal Tt(Main), Tt(Sub) of the steering torque sensorunit TS1, TS2, the second embodiment is characterized by the generationof an alternative signal upon detection of an abnormality in thesteering angle detection signal θs(Main), θs(Sub) of the steering anglesensor unit AS1, AS2.

The power steering device according to the second embodiment will beexplained below with reference to FIGS. 7 to 9. FIG. 7 is a flowchart ofan abnormal signal switching process. FIG. 8 is a control block diagramfor detection of the abnormality in the steering angle detection signal.FIG. 9 is a schematic view showing an example of calculation of anoperational steering angle signal.

As shown in FIG. 7, the processing of steps S6 and S8 to S11 of thesecond embodiment is the same as that of the first embodiment. Hence,explanation of the same processing of the second embodiment as that ofthe first embodiment will be omitted herefrom; and the followingexplanation will be focused only on the difference of the secondembodiment from the first embodiment.

At steps S1 b to S3 b, the steering angle detection signals θs(Main) andθs(Sub), the motor position detection signals θm(Main) and θm(Sub) andthe steering torque detection signals Tt(Main) and Tt(Sub) areretrieved.

In the second embodiment, the alternative signal is calculated as analternative to the steering angle detection signal θs(Main), θs(Sub).More specifically, calculated steering angle signals (alternativesignal) θss(Main) and θss(Sub) are obtained by first and second steeringangle calculation circuits 16 c and 16 d of the alternative signalcalculation section 20; and the calculated steering angle signalθss(Main) is outputted as the alternative signal by the alternativesignal determination circuit 18.

At step S4 b, the calculated steering angle signal (alternative signal)θss(Main) is obtained by the first steering angle calculation circuit 16c based on the steering torque detection signal Tt(Main), the motorposition detection signal θm(Main), the torsional rigidity Ktb of thetorsion bar and the reduction ratio Ng between the pillion shaft 2 andthe motor shaft.

A method of calculation of the calculated steering angle signal(alternative signal) θss(Main) will be explained below with reference toFIG. 9.

The rotational position θp of the pinion shaft 2 is determined bymultiplying the motor position detection signal θm by the reductionratio Ng between the pinion shaft 2 and the motor shaft. Further, thetorsion angle T/Ktb of the torsion bar is determined by dividing thesteering torque detection signal Tt by the torsional rigidity Ktb of thetorsion bar. There arises a difference between the steering angle andthe rotational angle θp of the pinion shaft 2 in an amount correspondingto the amount of torsion of the torsion bar. The calculated steeringangle signal (alternative signal) θss(Main) is thus obtained by addingthe torsion angle T/Ktb of the torsion bar to the rotational angle θp ofthe pinion shaft 2 as indicated by the following equation (2).

θss=θp+T/Ktb  (2)

In the case where the steering angle sensor AS is located on thesteerable wheel side with respect to the torsion bar, the calculatedsteering angle signal θss is equal to the rotational angle θp of thepinion shaft 2. There is no need to consider the torsion angle T/Ktb ofthe torsion bar.

Next, the calculated steering angle signal (alternative signal) θss(Sub)is obtained by the second steering angle calculation circuit 16 d basedon the steering torque detection signal Tt(Sub), the motor positiondetection signal θm(Sub), the torsional rigidity Ktb of the torsion barand the reduction ratio Ng between the pinion shaft 2 and the motorshaft.

A method of calculation of the calculated steering angle signal(alternative signal) θss(Sub) is similar to that of the calculatedsteering angle signal θss(Main).

At step S5 b, it is judged by an abnormality detection section 17 bwhether the difference |θs(Main)−θs(Sub)| between the two steering angledetection signals is smaller than an abnormality detection thresholdvalue. When the difference |θs(Main)−θs(Sub)| between the two steeringangle detection signals is judged as being smaller than the abnormalitydetection threshold value, the process goes to step S6. At step S6, theabnormality counter is cleared by the abnormality detection section. Atstep S7 b, the steering angle detection signal θs(Main) is set as thesignal for steering assist control by the switching judgment section 19.After that, the process ends the current control cycle.

As explained above, it is possible according to the second embodiment toobtain the same effects as according to the first embodiment.

Further, both of the steering angle detection signals θs(Main) andθs(Sub) are outputted as angular position signals of the steering shaft.It is thus possible for the abnormality detection section 17 to easilycompare these signals and achieve early and accurate detection of theabnormality.

In the case where the steering angle sensor unit AS1, AS2 is located onthe steering wheel side with respect to the torsion bar, there arises adifference between the steering angle and the motor position angle in anamount corresponding to the amount of torsion of the torsion bar. It isthus possible for the first, second steering angle calculation circuit16 c, 16 d to obtain the calculated steering angle signal (alternativesignal) θss(Main), θss(Sub) with higher accuracy by correcting the motorposition angle by an amount of such a difference.

Moreover, both of the steering angle and the motor position are angularposition information. In the case where the steering angle sensor unitAS1, AS2 is located on the steerable wheel side with respect to thetorsion bar, the calculated steering angle signal (alternative signal)θss(Main), θss(Sub) is obtained by correcting the motor positiondetection signal θm(Main), θm(Sub) by an amount corresponding to thereduction ratio of the reduction gear 5. It is thus possible to obtainthe calculated steering angle signal (alternative signal) θss(Main),θss(Sub) with high accuracy.

Third Embodiment

The third embodiment is characterized by the generation of analternative signal upon detection of an abnormality in the motorposition detection signal θm(Main), θm(Sub) of the motor position sensor61, 62.

The power steering device according to the third embodiment will beexplained below with reference to FIGS. 10 to 12. FIG. 10 is a flowchartof an abnormal signal switching process. FIG. 11 is a control blockdiagram for detection of the abnormality in the motor position detectionsignal. FIG. 12 is a schematic view showing an example of calculation ofan operational motor position signal.

As shown in FIG. 10, the processing of steps S6 and S8 to S11 of thethird embodiment is the same as that of the first embodiment. Hence,explanation of the same processing of the third embodiment as that ofthe first embodiment will be omitted herefrom; and the followingexplanation will be focused only on the difference of the thirdembodiment from the first embodiment.

At steps S1 c to S3 c, the motor position detection signals θm(Main) andθm(Sub), the steering angle detection signals θs(Main) and θs(Sub) andthe steering torque detection signals Tt(Main) and Tt(Sub) areretrieved.

In the third embodiment, the alternative signal is calculated as analternative to the detection signal of the motor position sensor 61, 62.More specifically, calculated motor position signals (alternativesignal) θms(Main) and θms(Sub) are obtained by first and second motorposition calculation circuits 16 e and 16 f of the alternative signalcalculation section 20; and the calculated motor position signalθms(Main) is outputted as the alternative signal by the alternativesignal determination circuit 18.

At step S4 c, the calculated motor position signal (alternative signal)θms(Main) is obtained by the first motor position calculation circuit 16e based on the steering torque detection signal Tt(Main), the steeringangle detection signal θs(Main), the torsional rigidity Ktb of thetorsion bar and the reduction ratio Ng between the pinion shaft 2 andthe motor shaft.

A method of calculation of the calculated motor position signal(alternative signal) θms(Main) will be explained below with reference toFIG. 12.

The steering torque detection signal Tt(Main) is divided by thetorsional rigidity Ktb of the torsion bar. The resulting division valueTt/Ktb is subtracted from the steering angle detection signal θs(Main).The resulting subtraction value is multiplied by the reduction ratio Ngbetween the pillion shaft 2 and the motor shaft. The calculated motorposition signal (alternative signal) θms(Main) is obtained by dividing 1by the resulting multiplication value Ng×(θs−T/Ktb). Namely, thecalculated motor position signal (alternative signal) θms is given bythe following equation (3).

θms=1/Ng×(θs−T/Ktb)  (3)

In the case where the steering angle sensor AS is located on thesteerable wheel side with respect to the torsion bar, the steering angledetection signal Os is equal to the rotational angle θp of the pillionshaft 2. There is no need to consider the value Tt/Ktb.

Next, the calculated motor position signal (alternative signal) θms(Sub)is obtained by the second motor position calculation circuit 16 f basedon the steering torque detection signal Tt(Sub), the steering angledetection signal θs(Sub), the torsional rigidity Ktb of the torsion barand the reduction ratio Ng between the pinion shaft 2 and the motorshaft.

A method of calculation of the calculated motor position signal(alternative signal) θms(Sub) is similar to that of the calculated motorposition signal (alternative signal) θms(Main).

At step S5 c, it is judged by an abnormality detection section 17whether the difference |θm(Main)−θm(Sub)| between the two motor positiondetection signals is smaller than an abnormality detection thresholdvalue. When the difference |θm(Main)−θm(Sub)| between the two motorposition detection signals is judged as being smaller than theabnormality detection threshold value, the process goes to step S6. Atstep S6, the abnormality counter is cleared by the abnormality detectionsection. At step 7 c, the motor position detection signal θm(Main) isset as the signal for steering assist control by the switching judgmentsection 19. After that, the process ends the current control cycle.

As explained above, it is possible according to the third embodiment toobtain the same effects as according to the first and secondembodiments.

Fourth Embodiment

The fourth embodiment is characterized by the execution of steeringassist control based on the alternative signal during a time period fromdetection of the abnormality to determination of the occurrence of theabnormality.

The power steering device according to the fourth embodiment will beexplained below with reference to FIG. 13. FIG. 13 is a flowchart of anabnormal signal switching process.

As shown in FIG. 13, the processing of steps S1 a to S11 of the fourthembodiment is the same as that of the first embodiment. Hence,explanation of the same processing of the fourth embodiment as that ofthe first embodiment will be omitted herefrom; and the followingexplanation will be focused only on the difference of the fourthembodiment from the first embodiment.

In the fourth embodiment, the abnormality counter is incremented at stepS11 when the abnormality counter is judged at step S8 as being smallerthan the predetermined value. At step S12, the first calculated steeringtorque signal (alternative signal) Tts(Main) or the second calculatedsteering torque signal Tts(Sub) is selected and set the signal forsteering assist control.

In the subsequent control cycle, the abnormality counter is cleared atstep S6 when the difference |Tt(Main)−Tt(Sub)| between the two steeringtorque detection signals is judged at step S5 a as being smaller theabnormality detection threshold value. At step S7 a, the control unit isswitched back to normal assist control based on the steering torquedetection signal Tt(Main).

Although the control unit is configured to detect the abnormality in thesteering torque detection signal Tt(Main), Tt(Sub) and generate thealternative signal in the fourth embodiment, it is alternativelyfeasible to detect an abnormality in the steering angle detection signalθs(Main), θs(Sub) or the motor position detection signal θm(Main),θm(Sub) and generate an alternative signal in the same manner as in thefourth embodiment.

As explained above, it is possible according to the fourth embodiment toobtain the same effects as according to the first embodiment. It is alsopossible to ensure steering safety at an early stage by switching overthe signal for steering assist control to the alternative signal beforedetermining the occurrence of the abnormality.

It is further possible to perform more appropriate steering controloperation by, when the sensor signal returns to the normal state,switching back to normal assist control.

In the fourth embodiment, the abnormality detection section 17 countsthe time for lapse of the predetermined time period and resets the timecount when the condition of detection of the abnormality by theabnormality detection section 17 is judged as being resolved so that,even in the case where the sensor signal once returns to the normalstate but goes back to the abnormality state, the abnormality detectionsection can recount the time for lapse of the predetermined time period.It is thus possible to improve the accuracy of judgement of theabnormality state.

Fifth Embodiment

The fifth embodiment is characterized by the execution of steeringassist control based on an average signal during a time period fromdetection of the abnormality to determination of the occurrence of theabnormality.

The power steering device according to the fifth embodiment will beexplained below with reference to FIGS. 14 and 15. FIG. 14 is aflowchart of an abnormal signal switching process. FIG. 15 is a timechart of the average signal.

As shown in FIG. 14, the processing of steps S1 a to S11 of the fourthembodiment is the same as that of the fourth embodiment. Hence,explanation of the same processing of the fifth embodiment as that ofthe first embodiment will be omitted herefrom; and the followingexplanation will be focused only on the difference of the fifthembodiment from the fourth embodiment.

After step S4 a, the average signal Tav is calculated at step S14. Inthe fifth embodiment, the average signal Tav is calculated as an averagevalue of the steering torque detection signals according to thefollowing equation: Tav=½×(Tt(Main)+Tt(Sub)) or calculated as an averagevalue of the steering torque detection signals and the calculatedsteering torque signals according to the following equation:Tav=¼×(Tt(Main)+Tt(Sub)+Tts(Main)+Tts(Sub)).

When the abnormality counter is judged at step S8 as being smaller thanthe predetermined value, the abnormality counter is incremented at stepS11. At step S14, the average signal Tav is selected and set as thesignal for steering assist control.

Although the control unit is configured to detect the abnormality in thesteering torque detection signal Tt(Main), Tt(Sub) and generate theaverage signal Tav in the fifth embodiment, it is alternatively feasibleto detect an abnormality in the steering angle detection signalθs(Main), θs(Sub) or the motor position detection signal θm(Main),θm(Sub) and generate an average signal Tav in the same manner as in thefifth embodiment.

It is possible according to the fifth embodiment to, even when thereoccurs a large change in one signal value in the occurrence of theabnormality, ease such a change by averaging a larger number of signalsas explained above. More specifically, the control unit is configuredto, when the abnormality occurs in the steering torque detection signalTt(Main), switch over to the average signal Tav, rather than, use theabnormal steering torque detection signal Tt(Main) as it is, forsteering assist control until determination of the occurrence of theabnormality. It is possible by this switching process to reduce a changein system behavior until the determination of the occurrence of theabnormality or at the switchover to the calculated steering torquesignal (alternative signal) Tts(Main), Tts(Sub) after determination ofthe abnormality and thereby possible to reduce handle shock at theswitchover.

Since the average signal calculated using the steering torque detectionsignals Tt(Main) and Tt(Sub) shows a value close to the actual steeringstatus, it is possible to perform more appropriate steering controloperation by properly using this average signal than by using only thecalculated steering torque signal Tts(Main), Tts(Sub) as the alternativesignal. Even though the steering torque detection signal Tt(Main),Tt(Sub) may contain an abnormality, it is possible to smoothen theabnormal signal and suppress the influences of the abnormal signal byaveraging the signals.

It is alternatively feasible to switch over the signal for steeringassist control to the average signal Tav at step 9 although the signalfor steering assist control is switched over to the calculated steeringtorque signal (alternative signal) Tts(Main) at step S9 afterdetermination of the abnormality in the fifth embodiment.

Sixth Embodiment

The sixth embodiment is characterized by the application of limitprocessing on the maximum motor torque during the execution of steeringassist control based on the alternative signal upon determining theoccurrence of the abnormality.

The power steering device according to the sixth embodiment will beexplained below with reference to FIGS. 16 to 19. FIG. 16 is a flowchartof an abnormal signal switching process. FIG. 17 is a control blockdiagram for detection of the abnormality. FIG. 18 is a schematic viewshowing one example of steering assist limit processing. FIG. 19 is aschematic view showing another example of steering assist limitprocessing.

As shown in FIG. 16, the processing of steps S1 a to S13 and S14 of thesixth embodiment is the same as that of the fifth embodiment. Hence,explanation of the same processing of the sixth embodiment as that ofthe first embodiment will be omitted herefrom; and the followingexplanation will be focused only on the difference of the sixthembodiment from the fifth embodiment.

Herein, a steering assist control section 26 is arranged to performhandle return control. Explanation of the steering assist controlsection 26 will be omitted herefrom because this section is not directlyrelevant to the present invention. Further, abnormality detectionsections 17 b and 17 c and switching judgment sections 19 b and 19 c arenot directly relevant to the sixth embodiment. Explanation of thesesections 17 b, 17 c, 19 b and 19 c will be omitted herefrom and will begiven in the following seventh embodiment.

When the occurrence of the abnormality is determined at step S8 byjudging that the abnormality counter exceeds the predetermined value,the alternative signal is selected and set as the signal for steeringassist control by a switching judgment section 19 a at step S9 so thatthe motor command current is determined based on the alternative signalwith reference to an assist map 21 in accordance with the vehicle speed.At step S15, limit processing is performed on the maximum torque of theelectric motor M by a limiter 23. Then, the motor command signal(voltage command signal, current command signal or the like) isoutputted by a motor control section 24 based on the output of thelimiter 23 so that electric motor M is driven via the inverter circuit12 according to the motor command signal.

As indicated by a solid line in FIG. 18, the maximum motor torque islimited to its maximum design value during normal steering assistcontrol. During the continuation of steering assist control by theswitchover to the alternative signal, the maximum motor torque islimited by the limiter 23 to a value smaller than the maximum designvalue as indicated by a broken line in FIG. 18.

It is possible according to the sixth embodiment to ensure steeringsafety by, when the occurrence of the abnormality is determined,performing assist limit processing as explained above.

As the steering assist force is lowered by the execution of assist limitprocessing, the steering force becomes heavier. This makes it easy forthe vehicle driver to recognize the occurrence of the abnormality.

In the assist map 21, the motor command current is normally setaccording to the steering torque at the respective vehicle speed asshown in FIG. 19. As shown in FIG. 19, the motor command current is setlarger when the vehicle speed is 0 km/h and becomes smaller as thevehicle speed is increased to 40 km/h, to 80 km/h and then to 120 km/h.Although the limiter 23 is utilized to perform limit processing on themaximum torque of the electric motor M during the continuation ofsteering assist control by the switchover to the alternative signal upondetermination of the abnormality in the sixth embodiment, it isalternatively feasible to perform steering assist limit processing byfixing the motor current command to a value corresponding to vehiclespeed=120 km/h as indicated by a broken line in FIG. 19.

Seventh Embodiment

The seventh embodiment is characterized by the application of redundantcomparison between the steering angle detection signals θs(Main) andθs(Sub) and the motor position detection signals θm(Main) and θm(Sub)used for the generation of the alternative signal.

The power steering device according to the seventh embodiment will beexplained below with reference to FIGS. 20 and 17. FIG. 20 is aflowchart of an abnormal signal switching process. FIG. 17 is a controlblock diagram for detection of the abnormality.

As shown in FIG. 20, the processing of steps S1 a to S13 and S14 of theseventh embodiment is the same as that of the sixth embodiment. Hence,explanation of the same processing of the seventh embodiment as that ofthe sixth embodiment will be omitted herefrom; and the followingexplanation will be focused only on the difference of the seventhembodiment from the sixth embodiment.

When the occurrence of the abnormality is determined at step S8, thealternative signal is selected and set as the signal for steering assistcontrol by the switching judgment section 19 a at step S9. At step S15,it is judged by the abnormality detection section 17 b whether thedifference |θs(Main)−θs(Sub)| between the steering angle detectionsignals is smaller than an abnormality detection threshold value.

When the difference |θs(Main)−θs(Sub)| between the steering angledetection signals is judged as being greater than or equal to theabnormality detection threshold value, the process goes to step S16. Atstep S16, it is judged by the abnormality detection section 17 b whetherthe steering angle abnormality counter is greater than or equal to apredetermined value.

When the steering angle abnormality counter is judged as being greaterthan or equal to the predetermined value, the process goes to step S17.At step S17, the steering assist control is interrupted by a fail-safejudgment section 13 a and a fail-safe processing section 16 b. At stepS18, the warning lamp is turned on so as to give a warning to thevehicle driver.

When the steering angle abnormality counter is judged as being smallerthan the predetermined value, the process goes to step S19. At step S19,the steering angle abnormality counter is incremented. At step S20, thewarning lamp is turned on so as to give a warning to the vehicle driver.

When the difference |θs(Main)−θs(Sub)| between the steering angledetection signals is judged at step S15 as being smaller than theabnormality detection threshold value, the process goes to step S21. Atstep S21, it is judged by the abnormality detection section 17 c whetherthe difference |θm(Main)−θm(Sub)| between the motor position detectionsignals is smaller than an abnormality detection threshold value. Whenthe difference |θm(Main)−θm(Sub)| between the motor position detectionsignals is judged as being greater than or equal to the abnormalitydetection threshold value, the process goes to step S22. At step 22, itis judged by the abnormality detection section 17 c whether the motorposition abnormality counter is greater than or equal to a predeterminedvalue.

When the motor position abnormality counter is judged at step S22 asbeing greater than or equal to the predetermined value, the process goesto step S23. At step S23, the steering assist control is interrupted. Atstep S24, the warning lamp is turned on so as to give a warning to thevehicle driver.

When the motor position abnormality counter is judged at step S22 asbeing smaller than the predetermined value, the process goes to stepS25. At step S25, the motor position abnormality counter is incremented.At step S26, the warning lamp is turned on so as to give a warning tothe vehicle driver.

When the difference |θm(Main)−θm(Sub)| between the motor positiondetection signals is judged at step S21 as being smaller than theabnormality detection threshold value, the process goes to step S27. Atstep S27, the steering angle abnormality counter and the motor positionabnormality counter are cleared. In this case, however, the steeringassist control is being executed according to the alternative signal asthe occurrence of the abnormality in the steering torque detectionsignal Tt(Main) or Tt(Sub) is determined. The warning lamp is thusturned on at step S10 so as to give a warning to the vehicle driver.

As explained above, it is possible to perform redundant monitoring onall of the sensor units and attain safety improvement by double failuredetection.

Eighth Embodiment

The eighth embodiment is characterized by the application of gradualdecrease of the steering assist force upon determining the occurrence ofthe abnormality in the steering torque detection signal Tt(Main) orTt(Sub).

The power steering device according to the eighth embodiment will beexplained below with reference to FIG. 21. FIG. 21 is a flowchart of anabnormal signal switching process.

As shown in FIG. 21, the processing of steps S1 a to S13 and S14 of theeighth embodiment is the same as that of the sixth embodiment. Hence,explanation of the same processing of the eighth embodiment as that ofthe sixth embodiment will be omitted herefrom; and the followingexplanation will be focused only on the difference of the eighthembodiment from the sixth embodiment.

When the occurrence of the abnormality is determined at step S8 byjudging that the abnormality counter exceeds the predetermined value,the alternative signal is selected and set as the signal for steeringassist control at step S9. At step S10, the warning lamp is turned on.At step S28, it is judged whether the gradual decrease counter isgreater than or equal to a predetermined value.

When the gradual decrease counter is judged as being smaller than thepredetermined value, the process goes to step S29. At step S29, gradualdecrease of the motor torque is caused. At step S30, the gradualdecrease counter is incremented. When the gradual decrease counterreaches the predetermined value, the motor torque is set to zero, or thesteering assist control is interrupted, at step S31.

When the difference |Tt(Main)−Tt(Sub)| between the steering torquedetection signals is judged at step S5 a as being smaller than theabnormality detection threshold value, the process goes to step S6. Atstep S6, the abnormality counter is cleared. Further, the gradualdecrease counter is cleared at step S32. The subsequent processing isthe same as that of the sixth embodiment.

As explained above, the control unit is configured to gradually decreasethe steering assist control after the expiration of a predetermined timefrom turn-on of the warning lamp upon detection of the abnormality inthe steering torque detection signal Tt(Main), Tt(Sub) in the eighthembodiment. It is possible by this control process to move the vehicleto a safe place before completely stopping the steering assist control.It is also possible to suppress continuous driving by the driver andattain safety improvement by gradually decreasing the steering assistamount with the passage of time. It is further possible to suppressuncomfortable steering feeling by gradually setting output limit.

Although the present invention has been described with reference to theabove specific embodiments, it is apparent to those skilled in the artthat: various modifications and variations of the embodiments are madewithin the technical spirit of the present invention; and suchmodifications and variations belong to the scope of the presentinvention.

For example, the motor control section may output the motor commandsignal for drive control of the electric motor based on the alternativesignal when the ignition switch of the vehicle is once turned off afterdetection of the abnormality by the abnormality detection section, andthen, turned on. In this case, it is possible to reduce the steeringload of the vehicle driver by continuing steering control operationbased on the alternative signal at a restart of the vehicle.

Hereinafter, technical ideas comprehended from the above embodiments butnot described in the following claims as well as effects thereof will beexplained below.

(a) The power steering device according to claim 2, wherein the motorcontrol section outputs the motor command signal for drive control ofthe electric motor based on the alternative signal when an ignitionswitch of a vehicle is once turned off after detection of theabnormality by the abnormality detection section, and then, turned on.

It is possible according to technical idea (a) to reduce a steering loadof a vehicle driver by continuing steering control operation based onthe alternative signal even at a restart of the vehicle.

(b) The power steering device according to claim 4, wherein the motorcontrol section outputs the motor command signal based on the comparedsignal when the abnormality detection section judges that a condition ofdetection of the abnormality is resolved after the motor control sectionoutputs the motor command signal based on the alternative signal upondetection of the abnormality by the abnormality detection section.

It is possible according to technical idea (b) to perform moreappropriate steering control operation by, when the sensor signalreturns to the normal state, switching back to normal control (i.e.control based on the compared signal).

(c) The power steering device according to technical idea (b), whereinthe abnormality detection section makes a time count for lapse of thepredetermined time period and, upon judging that the condition ofdetection of the abnormality is resolved before the lapse of thepredetermined time period, resets the time count.

It is possible according to technical idea (c) to, even in the casewhere the sensor signal once returns to the normal state but goes backto the abnormality state, restart the time count for lapse of thepredetermined time period and thereby possible to attain improvement inabnormality judgment accuracy.

(d) The power steering device according to claim 8, wherein, when theabnormality detection section detects the abnormality, the motor controlsection outputs the motor command signal such that the motor commandsignal gradually decreases.

It is possible according to technical idea (d) to suppress uncomfortablesteering feeling by gradually setting output limit.

(e) The control unit of the power steering device according to claim 10,wherein the abnormality detection section performs all of the firstcomparison, the second comparison and the third comparison and detectsthe abnormality in the torque sensor, the steering angle sensor and themotor position sensor.

It is possible according to technical idea (e) to attain safetyimprovement by performing redundant monitoring on all of the sensors.

(f) The control unit of the power steering device according to technicalidea (e), wherein the motor control section outputs the motor commandsignal for drive control of the electric motor based on the alternativesignal when an ignition switch of a vehicle is once turned off afterdetection of the abnormality by the abnormality detection section, andthen, turned on.

It is possible according to technical idea (f) to reduce a steering loadof a vehicle driver by continuing steering control operation based onthe alternative signal even at a restart of the vehicle.

(g) The control unit of the power steering device according to claim 10,wherein the abnormality detection section detects the abnormality anddetermines occurrence of the abnormality as determined information whena condition of detection of the abnormality continues for apredetermined time period; and wherein the motor control section outputsthe motor command signal based on the alternative signal before theabnormality detection section detects and determines the occurrence ofthe abnormality as the determined information.

It is possible according to technical idea (g) to ensure steering safetyat an early stage by switching over to the alternative signal beforedetermining the occurrence of the abnormality.

(h) The control unit of the power steering device according to technicalidea (g), wherein the motor control section outputs the motor commandsignal based on the compared signal when the abnormality detectionsection judges that the condition of detection of the abnormality isresolved after the motor control section outputs the motor commandsignal based on the alternative signal upon detection of the abnormalityby the abnormality detection section.

It is possible according to technical idea (h) to perform moreappropriate steering control operation by, when the sensor signalreturns to the normal state, switching back to normal control (i.e.control based on the compared signal).

(i) The control unit of the power steering device according to technicalidea (h), wherein the abnormality detection section makes a time countfor lapse of the predetermined time period and, upon judging that thecondition of detection of the abnormality is resolved before the lapseof the predetermined time period, resets the time count.

It is possible according to technical idea (i) to, even in the casewhere the sensor signal once returns to the normal state but goes backto the abnormality state, restart the time count for lapse of thepredetermined time period and thereby possible to attain improvement inabnormality judgment accuracy.

(j) The control unit of the power steering device according to claim 10,wherein the alternative signal is an average of the compared signal andthe compared signal equivalent value calculated based on any of thedetection signals other than the compared signal.

It is possible according to technical idea (j) to perform moreappropriate steering control operation by properly using the averagevalue than by using only the compared signal equivalent value as thealternative signal because the average signal contains in which thecompared signal itself is contained shows a value close to the actualsteering status. Even though the compared signal may contain anabnormality, it is possible to smoothen the abnormal signal and suppressthe influences of the abnormal signal by using the average value.

(k) The control unit of the power steering device according to claim 10,wherein the motor control section outputs the motor command signal suchthat the motor command signal is smaller when the abnormality detectionsection detects the abnormality than in the normal state.

It is possible according to technical idea (k) to attain safetyimprovement by setting output limit under the condition of detection ofthe abnormality.

(l) The control unit of the power steering device according to technicalidea (k), wherein, when the abnormality detection section detects theabnormality, the motor control section outputs the motor command signalsuch that the motor command signal gradually decreases.

It is possible according to technical idea (l) to suppress uncomfortablesteering feeling by gradually setting output limit.

1. A power steering device comprising: a steering system that steerssteerable wheels in response to steering operation of a steering wheel;an electric motor that applies a steering force to the steering system;an ECU that performs drive control of the electric motor; a torquesensor arranged on the steering system to detect a steering torquegenerated in the steering system; a steering angle sensor arranged onthe steering system to detect a steering angle as a steering amount ofthe steering wheel; and a motor position sensor arranged on the electricmotor to detect a rotor rotational position of the electric motor, theECU comprising: torque signal receivers that receive first and secondsteering torque detection signals from the torque sensor, said first andsecond steering torque detection signals being detected by and outputtedfrom different detection elements of the torque sensor or being detectedby a common detection element of the torque sensor and outputted throughdifferent electronic circuits; steering angle signal receivers thatreceive first and second steering angle detection signals from thesteering angle sensor, said first and second steering angle detectionsignals being detected by and outputted from different detectionelements of the steering angle sensor or being detected by a commondetection element of the steering angle sensor and outputted throughdifferent electronic circuits; motor position signal receivers thatreceive first and second motor position detection signals from the motorposition sensor, said first and second motor position detection signalsbeing detected by and outputted different detection elements of themotor position sensor or being detected by a common detection element ofthe motor position sensor and outputted through different electroniccircuits; an abnormal detection section that detects an abnormality inthe torque sensor, the steering angle sensor or the motor positionsensor by performing at least one of a first comparison between thefirst and second steering torque detection signals, a second comparisonbetween the first and second steering angle detection signals and athird comparison between the first and second motor position detectionsignals; an alternative signal calculation section that takes thedetection signal used for the at least one of the first, second andthird comparisons by the abnormal detection section as a compared signaland calculates a compared signal equivalent value, which is equivalentto the compared signal and in the same units as the compared signal, asan alternative signal using any of the detection signals other than thecompared signal or using the compared signal and any of the detectionsignals other than the compared signal; and a motor control section thatoutputs a motor command signal for drive control of the electric motorbased on the compared signal in a normal state where the abnormalitydetection section detects no abnormality in the torque sensor, thesteering angle sensor and the motor position sensor and, when theabnormality detection section detects the abnormality in the torquesensor, the steering angle sensor or the motor position sensor, outputsa motor command signal for drive control of the electric motor based onthe alternative signal.
 2. The power steering device according to claim1, wherein the abnormality detection section performs all of the firstcomparison, the second comparison and the third comparison and detectsthe abnormality in the torque sensor, the steering angle sensor and themotor position sensor.
 3. The power steering device according to claim2, wherein the motor control section outputs the motor command signalfor drive control of the electric motor based on the alternative signalwhen an ignition switch of a vehicle is once turned off after detectionof the abnormality by the abnormality detection section, and then,turned on.
 4. The power steering device according to claim 1, whereinthe abnormality detection section detects the abnormality and determinesoccurrence of the abnormality as determined information when a conditionof detection of the abnormality continues for a predetermined timeperiod; and wherein the motor control section outputs the motor commandsignal based on the alternative signal before the abnormality detectionsection detects and determines the occurrence of the abnormality as thedetermined information.
 5. The power steering device according to claim4, wherein the motor control section outputs the motor command signalbased on the compared signal when the abnormality detection sectionjudges that the condition of detection of the abnormality is resolvedafter the motor control section outputs the motor command signal basedon the alternative signal upon detection of the abnormality by theabnormality detection section.
 6. The power steering device according toclaim 5, wherein the abnormality detection section makes a time countfor lapse of the predetermined time period and, upon judging that thecondition of detection of the abnormality is resolved before the lapseof the predetermined time period, resets the time count.
 7. The powersteering device according to claim 1, wherein the alternative signal isan average value of the compared signal and the compared signalequivalent value calculated based on any of the detection signals otherthan the compared signal.
 8. The power steering device according toclaim 1, wherein the motor control section outputs the motor commandsignal such that the motor command signal is smaller when theabnormality detection section detects the abnormality than in the normalstate.
 9. The power steering device according to claim 8, wherein, whenthe abnormality detection section detects the abnormality, the motorcontrol section outputs the motor command signal such that the motorcommand signal gradually decreases.
 10. A control unit of a powersteering device, the power steering device comprising: a steering systemthat steers steerable wheels in response to steering operation of asteering wheel; an electric motor that applies a steering force to thesteering system; a torque sensor arranged to detect a steering torquegenerated in the steering system; a steering angle sensor arranged todetect a steering angle as a steering amount of the steering wheel; anda motor position sensor arranged to detect a rotor rotational positionof the electric motor, the control unit comprising: torque signalreceivers that receive first and second steering torque detectionsignals from the torque sensor, said first and second steering torquedetection signals being detected by and outputted from differentdetection elements of the torque sensor or being detected by a commondetection element of the torque sensor and outputted through differentelectronic circuits; steering angle signal receivers that receive firstand second steering angle detection signals from the steering anglesensor, said first and second steering angle detection signals beingdetected by and outputted from different detection elements of thesteering angle sensor or being detected by a common detection element ofthe steering angle sensor and outputted through different electroniccircuits; motor position signal receivers that receive first and secondmotor position detection signals from the motor position sensor, saidfirst and second motor position detection signals being detected by andoutputted different detection elements of the motor position sensor orbeing detected by a common detection element of the motor positionsensor and outputted through different electronic circuits; an abnormaldetection section that detects an abnormality in the torque sensor, thesteering angle sensor or the motor position sensor by performing atleast one of a first comparison between the first and second steeringtorque detection signals, a second comparison between the first andsecond steering angle detection signals and a third comparison betweenthe first and second motor position detection signals; an alternativesignal calculation section that takes the detection signal used for theat least one of the first, second and third comparisons by the abnormaldetection section as a compared signal and calculates a compared signalequivalent value, which is equivalent to the compared signal and in thesame units as the compared signal, as an alternative signal using any ofthe detection signals other than the compared signal or using thecompared signal and any of the detection signals other than the comparedsignal; and a motor control section that outputs a motor command signalfor drive control of the electric motor based on the compared signal ina normal state where the abnormality detection section detects noabnormality in the torque sensor, the steering angle sensor and themotor position sensor and, when the abnormality detection sectiondetects the abnormality in the torque sensor, the steering angle sensoror the motor position sensor, outputs a motor command signal for drivecontrol of the electric motor based on the alternative signal.
 11. Thecontrol unit of the power steering device according to claim 10, whereinthe abnormality detection section performs all of the first comparison,the second comparison and the third comparison and detects theabnormality in the torque sensor, the steering angle sensor and themotor position sensor.
 12. The control unit of the power steering deviceaccording to claim 11, wherein the motor control section outputs themotor command signal for drive control of the electric motor based onthe alternative signal when an ignition switch of a vehicle is onceturned off after detection of the abnormality by the abnormalitydetection section, and then, turned on.
 13. The control unit of thepower steering device according to claim 10, wherein the abnormalitydetection section detects the abnormality and determines occurrence ofthe abnormality as determined information when a condition of detectionof the abnormality continues for a predetermined time period; andwherein the motor control section outputs the motor command signal basedon the alternative signal before the abnormality detection sectiondetects and determines the occurrence of the abnormality as thedetermined information.
 14. The control unit of the power steeringdevice according to claim 13, wherein the motor control section outputsthe motor command signal based on the compared signal when theabnormality detection section judges that the condition of detection ofthe abnormality is resolved after the motor control section outputs themotor command signal based on the alternative signal upon detection ofthe abnormality by the abnormality detection section.
 15. The controlunit of the power steering device according to claim 14, wherein theabnormality detection section makes a time count for lapse of thepredetermined time period and, upon judging that the condition ofdetection of the abnormality is resolved before the lapse of thepredetermined time period, resets the time count.
 16. The control unitof the power steering device according to claim 10, wherein thealternative signal is an average value of the compared signal and thecompared signal equivalent value calculated based on any of thedetection signals other than the compared signal.
 17. The control unitof the power steering device according to claim 10, wherein the motorcontrol section outputs the motor command signal such that the motorcommand signal is smaller when the abnormality detection section detectsthe abnormality than in the normal state.
 18. The control unit of thepower steering device according to claim 17, wherein, when theabnormality detection section detects the abnormality, the motor controlsection outputs the motor command signal such that the motor commandsignal gradually decreases.